Method of processing a semiconductor wafer

ABSTRACT

A method of processing a semiconductor wafer that has circuits in each of a plurality of regions sectioned by a plurality of streets on the front surface and has a coating layer formed on the front surface having the circuits to a predetermined thickness, the method comprising a stress-reducing step of reducing the stress of the coating layer by forming a plurality of grooves in the coating layer formed on the front surface of the semiconductor wafer; and a grinding step of processing the back surface of the semiconductor wafer by grinding to a predetermined thickness after the stress-reducing step.

TECHNICAL FIELD

The present invention relates to a method of processing a semiconductorwafer having a large number of circuits on the front surface to apredetermined thickness.

BACKGROUND ART

In the process of semiconductor device production, semiconductor chipsare produced by forming a circuit such as IC, LSI or the like in aplurality of regions sectioned by a plurality of streets on the frontsurface of a substantially disk-like semiconductor wafer and dicingalong the streets. In order to improve the heat radiation of the thusmanufactured semiconductor chip, it is desired that the semiconductorchip be made as thin as possible. Also, to enable the downsizing of aportable telephone, smart card or personal computer that uses a largenumber of semiconductor chips, it is desired that the semiconductor chipbe made as thin as possible. To this end, before the semiconductor waferis divided into individual semiconductor chips by dicing, the backsurface of the semiconductor wafer is processed to a thickness of about50 μm by grinding.

When the back surface of the semiconductor wafer is ground to reduce itsthickness, both sides of the semiconductor wafer are warped toward thefront surface side. For example, when the back surface of asemiconductor wafer having a diameter of 200 mm is ground to a thicknessof 50 μm, both sides of the semiconductor wafer are warped toward thefront surface side by about 60 mm. It has been found that this warpingof the semiconductor wafer is caused by a coating layer formed byapplying a polyimide resin having excellent heat resistance, electricinsulating properties and mechanical strength to the front surface ofthe semiconductor wafer and baking it in order to protect circuitsformed on the front surface of the semiconductor wafer, or anothercoating layer such as a metal layer, insulating layer or the like. Thatis, a stress for pulling toward the center direction is generated in thecoating layer formed on the front surface of the semiconductor wafer.Therefore, when the back surface of the semiconductor wafer is processedto a thinness of about 50 μm thick by grinding, the semiconductor waferis warped as described above because its stiffness does not withstandthe above stress. When the semiconductor wafer is thus warped, it isdifficult to smoothly carry out dicing which is a subsequent step.

It is a principal technical subject of the present invention that hasbeen made in view of the above fact to provide a method of processing asemiconductor wafer, which can inhibit the semiconductor wafer frombeing warped even when its back surface is ground to be processed thin.

DISCLOSURE OF THE INVENTION

To attain the above principal technical subject of the presentinvention, according to the present invention, there is provided amethod of processing a semiconductor wafer that has circuits in aplurality of regions sectioned by a plurality of streets on the frontsurface and has a coating layer formed on the front surface having thecircuits to a predetermined thickness, the method comprising:

a stress-reducing step of reducing the stress of the coating layer byforming a plurality of grooves in the coating layer formed on the frontsurface of the semiconductor wafer; and

a grinding step of processing the back surface of the semiconductorwafer by grinding to a predetermined thickness after the stress-reducingstep.

The plural grooves to be formed in the coating layer in the abovestress-reducing step are formed along a plurality of streets.

The plural grooves to be formed in the coating layer in the abovestress-reducing step are formed by cutting by moving the semiconductorwafer relatively to the cutting blade while the cutting blade isrotated.

BRIEF DESCRIPTION OF THE INVENTION

FIGS. 1(a) and (b) are explanatory diagrams showing a semiconductorwafer to be processed according to the present invention;

FIGS. 2(a) and (b) are explanatory diagrams showing the stress-reducingstep of the processing method of the present invention;

FIGS. 3(a) and (b) are explanatory diagrams showing the step of affixinga protective tape to the front surface of a semiconductor wafer afterthe stress-reducing step shown in FIGS. 2(a) and (b); and

FIG. 4 is an explanatory diagram showing the cutting step of theprocessing method of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The method of processing a semiconductor wafer according to anembodiment of the present invention will be described in detailhereinafter with reference to the accompanying drawings.

FIGS. 1(a) and (b) show a semiconductor wafer to be processed accordingto the present invention. FIG. 1(a) is a perspective view of thesemiconductor wafer and FIG. 1(b) is a partially enlarged sectional viewof the semiconductor wafer. The semiconductor wafer 2 shown in FIGS.1(a) and (b) has a diameter of 200 mm and a thickness of 500 μm, and acircuit 22 is formed in each of a plurality of regions sectioned by aplurality of streets 21 on the front surface of the semiconductor wafer2. A coating layer 23 as a protective film is formed on the frontsurface having the circuits 22. In the illustrated embodiment, thecoating layer 23 is formed by applying a polyimide resin to the frontsurface of the semiconductor wafer 2 including the circuits 22 andbaking it, and has a thickness of about 10 μm.

The back surface of the above-described semiconductor wafer 2 is groundto process it to a thickness of about 50 μm before it is divided intoindividual semiconductor chips by dicing along the plurality of streets21. When the semiconductor wafer 2 is processed thin, it is warped bythe influence of stress generated in the coating layer 23 as describedabove. In the present invention, therefore, to inhibit the occurrence ofthe above warping, the step of reducing stress that has generated in theabove coating layer 23, for pulling toward the center direction iscarried out before the back surface of the semiconductor wafer 2 isground.

FIGS. 2(a) and (b) show the stress-reducing step of reducing the stressof the coating layer 23. FIG. 2(a) is a perspective view of the step andFIG. 2(b) is a partially enlarged sectional view of the semiconductorwafer 2. In the stress-reducing step, as shown in FIGS. 2(a) and (b),grooves 231 are formed in the coating layer formed on the front surfaceof the semiconductor wafer 2. A cutting machine that is generally usedas a dicing machine can be used for forming the grooves 231. That is,the grooves 231 are formed in the coating layer 23 by holding thesemiconductor wafer 2 on a chuck table 31 of the cutting machine andcut-feeding the chuck table 31 in a direction shown by an arrow X whilea cutting blade 32 is rotated. It is desired that the grooves 231 beformed along the streets 21 in order not to damage the circuits 22. Thedepth of the grooves 231 is equivalent to the thickness of the coatinglayer 23 so that the coating layer 23 is desirably cut. The grooves 231do not need to be always formed along all the streets, and are enough tobe formed to such an extent that the stress of the coating layer 23 canbe reduced and warping which occurs when the semiconductor wafer 2 ismade as thin as about 50 μm can be suppressed. For example, four grooves231 may be formed per each direction, though the grooves 231 may beformed along all the streets. As to cutting conditions for forming thegrooves 231, it is appropriate that the revolution speed of the cuttingblade 32 is about 20,000 rpm and the feed speed of the chuck table 31 isabout 50 to 100 mm/sec.

After the stress-reducing step terminates, the semiconductor wafer 2 isturned upside down as shown in FIG. 3(a), and a protective sheet 4 forgrinding is affixed on the front side (on the circuit 22-formed side),as shown in FIG. 3(b).

The grinding step for processing the back surface of the semiconductorwafer 2 having the protective sheet 4 affixed on the front surface asdescribed above to a predetermined thickness is then carried out. Thatis, as shown in FIG. 4, the semiconductor wafer 2 is held on the chucktable 51 of the grinding machine 5 in such a manner that the backsurface faces up, and a grinding wheel 52 is rotated at a revolutionspeed of 6,000 rpm while the chuck table 51 is rotated at a revolutionspeed of 300 rpm, and is brought into contact with the back surface ofthe semiconductor wafer 2 to grind the back surface of the semiconductorwafer 2. The semiconductor wafer 2 is ground until its thickness becomesabout 50 μm. Even when the back surface of the semiconductor wafer 2 isground until its thickness becomes about 50 μm, the occurrence ofwarping is inhibited because the stress of the coating layer 23 has beenreduced by forming a plurality of grooves 231 in the coating layer 23formed on the front surface of the semiconductor wafer 2 in the abovestress-reducing step.

The semiconductor wafer 2 whose back surface has been processed to athickness of about 50 μm as described above is conveyed to thesubsequent dicing step where it is cut along the streets 21 by a dicingmachine such as a cutting machine or the like and divided intoindividual semiconductor chips.

INDUSTRIAL UTILIZATION FEASIBILITY

In the method of processing a semiconductor wafer to a predeterminedthickness according to the present invention, the stress-reducing stepof reducing the stress of a coating layer by forming a plurality ofgrooves in the coating layer formed on the front surface of thesemiconductor wafer is carried out before the back surface of thesemiconductor wafer is ground. Therefore, even when the back surface ofthe semiconductor wafer is ground thin, the occurrence of warping isinhibited as the stress of the coating layer has been reduced.Therefore, the dicing work that is the subsequent step can be carriedout smoothly.

1. A method of processing a semiconductor wafer that has circuits in aplurality of regions sectioned by a plurality of streets on the frontsurface and has a coating layer formed on the front surface having thecircuits to a predetermined thickness, the method comprising: astress-reducing step of reducing the stress of the coating layer byforming a plurality of grooves in the coating layer formed on the frontsurface of the semiconductor wafer; and a grinding step of processingthe back surface of the semiconductor wafer by grinding to apredetermined thickness after the stress-reducing-step.
 2. The method ofprocessing a semiconductor wafer according to claim 1, wherein theplural grooves to be formed in the coating layer in the stress-reducingstep are formed along the plurality of streets.
 3. The method ofprocessing a semiconductor wafer according to claim 1, wherein theplural grooves to be formed in the coating layer in the abovestress-reducing step are formed by cutting by moving the semiconductorwafer relatively to the cutting blade while the cutting blade isrotated.